Apparatus for crucible-free zone-melting of crystalline materials

ABSTRACT

Apparatus for crucible-free zone melting a rod-shaped member of crystalline material includes at least one heating device for heating a first portion of the rod-shaped member to the melting point thereof so as to form a melting zone therein, and another heating device for afterheating a second portion of the rodshaped member directly adjacent to the melting zone and recrystallized therefrom, the other heating device comprising a ring-shaped heat radiator surrounding the portion of the rodshaped member adjacent the melting zone, the radiator being heatable to a temperature nearly equal to the melting temperature of the crystalline material.

United States Patent Keller [54] APPARATUS FOR CRUClBLE-FREE ZONE-MELTING OF CRYSTALLINE MATERIALS [72] Inventor: Wolfgang Keller, Pretzfeld, Germany [73] Assignee: Siemem-Schuckertwerke Aktlengesellschaft, Berlin, Germany [22] Filed: June 20, 1969 [21] App]. No.: 838,026

Related US. Application Data [63] Continuation of Ser. No. 564,232, July 11, 1966,

abandoned.

[30] Foreign Application Priority Data July 10, 1965 Germany ..S 98119 Apr. 26, 1966 Germany ..S 103418 [52] US. Cl ..23/273 SP [51] Int. Cl. ..B0lj 17/10 [58] Field of Search ..23/273 SP, 301 SP [56] References Cited UNITED STATES PATENTS 2,932,562 4/1960 Pfann ..,...23/273 1 Mar. 14, 1972 3,121,619 2/1964 Scholte 23/273 3,258,314 6/1966 Redmond et al. ...23/301 3,271,115 9/1966 Keller ..23/273 FOREIGN PATENTS OR APPLICATIONS 674,121 11/1963 Canada ..23/301 Primary ExaminerNorman Yudkoff Assistant Examiner-S. Silverberg Attorney-Curt M. Avery [5 7] ABSTRACT 5 Claims, 4 Drawing Figures PYROLITIC HARD CARBON L AYE R PATENTEDMAR 14 I972 3,649,210

RI NB PYROLITIC HARD CARBON L AYE R PYROLITIC HARD CARBON LAYER SLIPPAGES AAAAAAAAA Fi 3 APPARATUS FOR CRUCIBLE-FREE ZONE-MELTIN G F CRYSTALLINE MATERIALS This application is a continuation of Ser. No. 564,232 filed July 11, 1966 now abandoned.

It is already known, in crucible-free zone melting apparatus, to provide a heating device for preheating and/or afterheating the solidified material, which is being processed, in addition to the heating device for producing the melting zone. Such additional heating device serves for permitting the temperature gradients to be as level and uniform as possible in the solidified semiconductor material extending from the melting zone to the end of the rod, whereby the crystalline quality thereof is improved. Induction-heating coils and heat radiators are particularly adaptable for use as these additional heating devices as well as for the heating device employed to melt the rod and form the melting zone therein.

It is an object of my invention to provide an improved device for crucible-free zone melting of crystalline materials, by means of which a reduction in the crystal dislocations is achieved, such as would not be achievable in any other manner.

With the foregoing and other objects in view, I provide apparatus for crucible-free zone melting of rod-shaped members consisting of crystalline material, comprising at least one heating device for heating a portion of the rod-shaped member to the melting point thereof, and another heating device for afterheating the portion of the rod-shaped member crystallizing out of the melt. The second heating device is in the form of a ring-shaped heat radiator surrounding the rod-shaped member, which is heated to a temperature that is nearly equal to the melting temperature of the material being processed.

More specifically, in accordance with my invention, the ring-shaped heat radiator or radiant heating ring is in the form of a hollow cylinder having a longitudinal dimension which is substantially equal to the length of the melting zone. A further feature of my invention is that the radiant heating ring consists of graphite and has an inner diameter which is substantially double the diameter of the rod-shaped member.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in device for crucible-free zone melting crystalline materials, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIG. 1 is a longitudinal schematic view of the device constructed in accordance with my invention;

FIGS. 2 and 3 are diagrammatic views of'so-called lineage and slippage etch pit patterns respectively; and

FIG. 4 is a further embodiment of the device shown in FIG. 1 which is particularly suitable for zone melting relatively thick rods.

In the interest of clarity, only those structural features which are pertinent to the invention are shown in the drawing.

Referring now to the drawings and first particularly to FIG. 1 thereof, there is shown a semiconductor rod 2, consisting of silicon, for example, wherein a melting zone 3 is produced with the aid of a heat-producing member 4 which, as shown, is in the form of an induction-heating coil. Due to relative motion between the rod 2 which is being processed and the heatproducing member 4, the melting zone 3 is passed through the rod 2 in the longitudinal direction thereof. If a monocrystalline seed crystal is fused to the end of the rod from which the melting zone pass begins, the rod finally formed by the zonemelting method in this manner is monocrystalline. By repeating the melting zone pass through the rod many times over, a purifying effect can be obtained; through zone levelling, the

impurities contained in the semiconductor rod can be distributed uniformly over the rod length and rod cross section.

In addition to the purity of the material being processed, particularly semiconductor material for use as electronic components, the crystal quality of the material is also of particular significance. In connection therewith, the uniform distribution of the dislocations over the rod cross section and over the rod length is of importance in addition to the absolute value of the dislocation density. Such dislocations can be rendered visible by providing samples of the material havingground surfaces which are then etched in a predetermined manner whereby the dislocations are visible as etch pits. Crystal imperfections in the form of serially aligned dislocations, for example in the form of lineages as shown in FIG. 2, and in the form of slippages as shown in FIG. 3, are particularly harmful.

It has been found that the serially aligned dislocations can be removed and that the absolute number of dislocations can be reduced by afterheating the semiconductor rod. The serially aligned dislocations in the form of lineages have presented great difficulties particularly and have not until now been able to be removed or reduced to a measurable degree.

By means of the apparatus of the invention in the instant application, it is now possible also to fully remove the serially aligned dislocations in the form of lineages as shown in FIG. 2. Moreover, a sharp reduction of the absolute dislocation density is also achieved.

In experiments which I have conducted, I have employed a graphite ring 5 (FIG. 1) in the form of a hollow cylinder for afterheating the recrystallized rod of semiconductor material. The graphite ring can either be directly heated by the passage of current therethrough or can be heated inductively. The temperature of the graphite ring was kept above the melting point of the semiconductor material which was being processed, in the case of silicon (melting point 1,420 C.), it was maintained at a temperature of l,500 C., for example. The height or longitudinal dimension of the cylindrical heat radiator 5 was substantially the same as the length of the melting zone 3. In this way, a particularly uniform temperature gradient from the melting zone into the resolidified semiconductor material is produced which is of particular importance in the region of the resolidifying rod directly adjacent to the melting zone. Based upon experience, I have found that in this region of the recrystallized rod, dislocations are fomied because the semiconductor material is still to a great extent in the plastic state.

Another embodiment of the apparatus constructed in accordance with my invention, which is especially suitable for zone-melting thick ro ds, is shown in FIG. 4. Generally, thick rods cannot be melted by induction-heating coils whose inner diameter is greater than the diameter of the rod, without resulting in the dripping of some of the molten material from the melting zone. Consequently, heating coils whose inner diameter is smaller than that of the rod diameter have been used, and the zone-melting process has been carried out so that first a thin seed crystal 7 is fused to the rod and then, by compressing the melt, the recrystallizing rod portion is increased in thickness to the desired cross section which can, for example, be the same as that of the rod portion which is being supplied to the melt. During the further course of the zonemelting operation, the cross section of the recrystallizing rod is maintained substantially constant. Due to the fact that the melt is chocked in the vicinity of the heating coil and is supported by the magnetic field of the heating coil, the material of the melt is prevented from dripping, yet the crystal quality is reduced due to the highly nonuniform temperature gradients which are formed in the recrystallizing zone of the rod. With the device of the invention in the instant application, however, rod-shaped monocrystals of large cross section having good crystal quality can be obtained.

The radiant heating ring 5 should have an inner diameter considerably larger than the outer diameter of the rod being processed, that is, twice as large or larger. It is essential that the radiation heating ring exert'no damaging effects upon the semiconductor material; it must consequently consist of highly pure material such as graphite, for example, which is suitable for this purpose. Tungsten or molybdenum can also be used. The radiation heating ring is advantageously heated to an elevated temperature in vacuum before its use in the zonemelting apparatus whereby impurities, especially on the surface thereof, which would otherwise vaporize from the radiation heating ring and penetrate into the semiconductor material, can be removed. The semiconductor material on the side of the rod which is being melted can also be additionally heated, for example with the aid of a preheating coil 6 located above the heating coil 4 which serves to produce the melting zone. This feature also promotes the equalization of the temperature of the melt and thereby the maintenance of the growing boundary surface or freezing front in a single plane.

As determined by experiments which I have conducted, the dislocation density is reduced when a radiation heating ring of the aforementioned type is employed. Thus, without employing the radiant heating ring, all other conditions being equal, dislocation densities of substantially 120,000 etch pits per cm. are obtained whereas, under the very same conditions, the etch-pit density can be reduced to less than 20,000 per cm., for example 9,000 or 12,000, by employing the radiant heating ring 5. In addition, as stated hereinbefore, the serially aligned dislocations in the form of lineages, as shown by etchpit arrays, are completely removed.

It has been found that at elevated temperatures, in addition to the vaporization of impurities from the radiation heating ring, surface particles can also chip off therefrom. These disadvantages can be avoided if the radiation heating ring is provided with a thick layer of a material which has a coefficient of expansion substantially the same as that of the base material on which it is coated, is stable to a temperature of 1,600 C. and can be produced with a great degree of purity. For a radiant heating ring of graphite, a layer of carbon which is deposited by pyrolysis, for example, of a cyclical hydrocarbon compound, and which is known as hard carbon, has proven to be successful.

I claim:

1. Apparatus for crucible-free zone melting a rod-shaped member of crystalline material, comprising at least one heating device substantially coaxially surrounding the rod-shaped member for heating a first portion of the rod-shaped member to the melting point thereof so as to form a melting zone therein, and another heating device for afterheating a second portion of the rod-shaped member directly adjacent to said melting zone and recrystallized therefrom, said other heating device comprising a ring-shaped heat radiator formed of graphite and substantially coaxially surrounding the portion of the rod-shaped member adjacent said melting zone, and means for heating said radiator to a temperature greater than the melting temperature of the crystalline material and at most exceeding the melting temperature to such extent that the molten zone remains located outside the portion of the rodshaped member surrounded by the heat radiator.

2. Apparatus according 0 claim 1, wherein said radiator is in the form of a hollow cylinder having a longitudinal dimension substantially equal to the longitudinal dimension of the melting zone.

3. Apparatus according to claim 1, wherein said ring-shaped heat radiator has an inner diameter at least double the diame' ter of the rod-shaped member.

4. Apparatus according to claim 1, wherein the inner diameter of said one heating device is less than the diameter of the rod-shaped member.

5. Apparatus according to claim 1, wherein said ring'shaped heat radiator is coated with a layer of highly pure material stable up to a temperature of ],600 C. and having substantially the same thermal coefficient of expansion as the base material of said ring-shaped heat radiator.

6. Apparatus according to claim 5, wherein said layer consists of pyrolytic hard carbon. 

2. Apparatus according o claim 1, wherein said radiator is in the form of a hollow cylinder having a longitudinal dimension substantially equal to the longitudinal dimension of the melting zone.
 3. Apparatus according to claim 1, wherein said ring-shaped heat radiator has an inner diameter at least double the diameter of the rod-shaped member.
 4. Apparatus according to claim 1, wherein the inner diameter of said one heating device is less than the diameter of the rod-shaped member.
 5. Apparatus according to claim 1, wherein said ring-shaped heat radiator is coated with a layer of highly pure material stable up to a temperature of 1,600* C. and having substantially the same thermal coefficient of expansion as the base material of said ring-shaped heat radiator.
 6. Apparatus according to claim 5, wherein said layer consists of pyrolytic hard carbon. 